Ink jet heads that record information by using the piezoelectric effect of a piezoelectric element have been known in the art.
An ink jet head of this type includes piezoelectric actuators including a common electrode, piezoelectric elements and separate electrodes deposited in this order, and an ink channel substrate in which pressure chambers are formed. A vibration plate is provided on one side of the piezoelectric actuators. The vibration plate is bonded onto the ink channel substrate via an adhesive. When discharging ink, a voltage is applied between the common electrode and the separate electrode, thereby contracting/expanding the piezoelectric element. The contraction/expansion is restricted by the vibration plate, whereby the piezoelectric actuator undergoes flexural deformation in the thickness direction. The flexural deformation changes the volume of the pressure chamber, whereby ink is discharged out of the pressure chamber through a nozzle.
The piezoelectric actuator described above includes a displacement region and a wiring region. The displacement region is located corresponding to the pressure chamber. The wiring region is a region other than the displacement region and is a region where an electrode line is provided for connecting the separate electrode with a driving circuit for applying a voltage between the electrodes.
It is common for a piezoelectric actuator as described above that a single piece of piezoelectric element is deposited across the displacement region and the wiring region for reasons such as to simplify the manufacture of the ink jet head. Moreover, the dielectric constant tends to be high when one attempts to obtain a high piezoelectric characteristic, and the piezoelectric element in the displacement region is required to have a high piezoelectric characteristic in order to improve the ink discharging performance. Therefore, a piezoelectric element having a high dielectric constant is deposited across the displacement region and the wiring region. In such a case, however, the piezoelectric element has a high dielectric constant not only in the displacement region but also in the wiring region, where a high piezoelectric characteristic is not required. Then, the electrostatic capacity between the electrodes in the wiring region is high, whereby a high voltage needs to be applied between the electrodes for discharging ink. As a result, an excessive load is placed upon the driving circuit.
One possible way to solve this problem may be to deposit a piezoelectric element having a low dielectric constant across the displacement region and the wiring region. In such a case, however, the piezoelectric element has a low dielectric constant not only in the wiring region but also in the displacement region. Then, the piezoelectric characteristic of the piezoelectric element in the displacement region decreases, which is not preferred for improving the ink discharging performance.
In view of this, Japanese Laid-Open Patent Publication No. 9-156099 discloses an ink jet head including a piezoelectric film layer between a piezoelectric element in the wiring region and an upper electrode, the piezoelectric film having a lower dielectric constant than that of the piezoelectric element. In this way, it is possible to decrease the overall dielectric constant of the piezoelectric element while preventing a decrease in the piezoelectric characteristic of the piezoelectric element in the displacement region.
In this ink jet head, however, a step occurs between the upper electrode in the displacement region and the upper electrode in the wiring region, whereby the upper electrode is likely to break. Moreover, since the piezoelectric characteristic of the piezoelectric element in the wiring region is different from that of the piezoelectric film, the piezoelectric element and the film are likely to be peeled off from each other.
Thus, development of a new technique has been called for in the art that is free of such a problem as described above and yet allows for a decrease in the overall dielectric constant of the piezoelectric element while preventing a decrease in the piezoelectric characteristic of the piezoelectric element in the displacement region.